It is known that serotonin is contained abundantly in platelets, a blood component, and that upon stimulation by thromboxane A.sub.2, ADP, collagen or the like, it is released to synergistically act on the release of various platelet aggregation factors through activation of the serotonin-2 receptor in platelets and vascular smooth muscle cells and on vasoconstriction by norepinephrine through the .alpha..sub.1 receptor, thereby inducing strong platelet aggregation and vasoconstriction [P. M. Vanhoutte, "Journal of Cardiovascular Pharmacology", Vol. 17 (Supple. 5), S6-S12 (1991)].
With the foregoing in view, there is hence an outstanding desire for the development of a serotonin-2 receptor antagonist as a circulatory disease therapeutic for preventing thrombus formation and vasoconstriction so that the serotonin-2 receptor antagonist can be used for hypertension and ischemic heart diseases such as angina pectoris, myocardial infarction, heart failure and post-PTCA restenosis. It is however the present situation that no drug has been obtained yet with sufficient antagonism and selectivity. Pharmaceuticals having .alpha..sub.1 -blocking action in combination with anti-serotonin action are expected to reduce side effects caused by hypotensive action based on .alpha..sub.1 -blocking action, such as orthostatic disorder and reflex tachycardia, so that some drugs having both actions have been developed. However, none of them have been provided with sufficient hypotensive action.